The present invention is directed to the fermentation of acetic acid by the aerobic oxidation of ethanol. Specifically, the present invention is directed to an apparatus and process for the oxidative acetic acid fermentation of an unpreserved wine.
The commercial production of vinegar, which is an acetic acid containing liquid from wines is of widespread use. The typical process of fermenting wines involves the aerobic oxidation of the ethanol to acetic acid. This aerobic oxidation of ethanol is carried out in most commercial processes by acetic acid bacteria, such as those of the genera acetobacter. These types of bacteria naturally occur on various surfaces of plants, such as flowers and fruits. These types of bacteria develop as a secondary microflora on decomposing plant material following the conversion of sugars to ethanol by yeasts. The acetic acid bacteria utilize ethanol as a substrate upon which to grow.
The viability of the acetic acid bacteria is dependent upon not only the presence of a sufficient concentration of ethanol but also upon a sufficient concentraton of oxygen. If either the ethanol or the oxygen become limited the viability of these types if bacteria is seriously impaired. Furthermore, these types of bacteria are also highly temperature sensitive, which affects their productivity and the coversion of ethanol to acetic acid. Thus any commercial process which utilizes these acetic acid bacteria must take into account these requirements.
One particularly widely used commercial process involves the batch preparation of vinegar, an acetic acid containing liquid from wine. In a batch operation, the wine is charged into a vat. Typically, the quantity of wine which will undergo fermentation is anywhere from 3000 to 20,000 liters. As already stated, one of the requirements of the aerobic bacteria is the concentration of available oxygen. A batch type operation requires that air or other oxygen-containing gas be pumped into the liquid. This requirement for pumping an oxygen-containing gas into the liquid is necessitated because the transportation of gas across the liquid-air boundary of a large volume of wine in the vat is not sufficient for purposes of maintaining a viable bacterial culture.
Furthermore, the aerobic oxidation of ethanol to acetic acid by the bacteria is a highly exothermic process. As stated, acetic acid bacteria are highly temperature sensitive. It has thus been found in the commercial production of acetic acid that the fermentating wine must be sufficiently cooled so as to maintain the viability of bacteria.
Examples of presently available commercial type fermentators are disclosed in U.S. Pat. Nos. 2,997,424, issued to Mayer; 3,014,804, issued to ELs et al; 3,252,870, issued to Braun et al; 3,445,245, issued to Ebner; 3,531,373, issued to Eder; 3,681,200 issued to Ridgway; and 4,569,845 issued to Nodes. Generally, such disclosed apparatus inject air into the liquid using a rotating impeller type mechanism which is located at the bottom of the fermentation vessel. These rotating impeller mechanisms not only introduce air into the wine but establish an upperly spiralling rotation in the wine. This spiralling movement in the wine mixes and disperses the air into the wine.
As illustrated in some of the above identified patent references, a mechanism is typically positioned in the fermentation vessel to cool the wine during the fermentation process which, as stated, is exothermic. The most typically utilized mechanism is a spirally configured pipe through which a cooling wine is directed. The spiral of the pipe compatibly conforms to the spiralling movement of the wine created by the rotating impeller aerator. Thus the overall spiralling movement of the wine is not impaired by the cooling mechanism.
A major disadvantage with these types of acetic acid fermentators is the limited degree of mixing due to only creating a spiralling circulation of the wine. It has been found that this uniform spiralling circulation of the wine does not thoroughly disperse the air. This is due to the presence of areas of low circulation found outside the upperly spiralling circulation area of the wine. This establishes areas of low air concentration which may be less than necessary to sustain the viability of the bacteria. If the viability of the bacteria becomes seriously impaired, death of such bacteria will occur. The death of bacteria creates a potential for the production of foam at the surface of the wine. The establishment and production of this foam is highly undesirable since it impairs the overall quality of the produced acetic acid liquid, and impairs the overall efficiency of the fermentating process.
In order to compensate for the production of foam by this inefficient aeration of the wine, many commercial devices utilize defoamers. These defoamers continually skim or remove in some manner the foam being generated at the surface of the wine. Examples of such foam removal devices are disclosed in the Ebner (3,445,245) and Nodes (4,569,845) patent references.
Another disadvantage with presently available fermentators is the manner by which the fermentation process is initiated. Generally, the processes disclosed by the above referred to patents require the addition to the wine of a separately prepared culture of the desirable acetic acid bacteria. This prepared culture is used to seed the fermentation reaction in the vessel. Typically, the fermentating vessel is charged with a desirable amount of a wine and a seed bacteria culture. The operation of the process is then initiated by aerating and regulating the temperature of the wine to maintain the viability of the bacteria added to the wine.
Some of the processes disclosed by the above referred to patent references then utilize the fermenting wine as a seed culture for subsequent fermentations. In other words, after the fermentation of the initial wine has been completed, a quantity of the now fermented wine is discharged from the fermentation vessel. This discharged quantity is slightly less than the initial quantity charged to the vessel. This leaves a small volume of the already fermented wine in the vessel. More wine is then charged to the vessel with the already present fermented wine now acting as the seed culture. In this manner, successive fermentations may be carried out after a seed culture liquid has been established in the fermentator.
The use of the independently prepared seed culture of a desired bacteria is costly and time consuming. Furthermore, use of a seed culture, including the use of a previously fermented wine as the seed culture, for each successive fermentation process requires that in each succeeding fermentation the concentration of bacteria in the wine must first be sufficiently increased before a substantial conversion of the ethanol to acetic acid will occur. This initial start-up requires a substantial amount of time until the desired concentration of the bacteria is established in the liquid to efficiently carry out the fermentation process.
Another disadvantage with the presently available fermentators is in the loss of ethanol and acetic acid during the fermentation process. As already discussed, it is essential that the bacteria in the fermentating wine be exposed to a sufficient quantity of air. This is achieved not only by the aeration but by the mixing of the liquid in the fermentator. As the wine is more violently mixed and/or the amount of air being introduced is increased, there will be evaporation of this air at the surface of the wine. Furthermore, as the air passes through the fermenting wine, it will pick up ethanol and/or actic acid. Thus the air passing through and evaporating off the wine carries with it ethanol and/or acetic acid.
All of the disclosed fermentators vent the exhaust gas out of the fermentator. This exhaust gas thus carries with it some ethanol and/or acetic acid which reduces the overall efficiency of converting the initial concentration of ethanol to acetic acid. As will be more fully discussed herein, it has now been determined that this loss of ethanol, and thus acetic acid in the final wine affects the quality of the fermentated liquid produced, i.e. vinegar. It has been determined that the overall organoleptic characteristics, as well as the taste of the resulting fermentated vinegar, is impaired by this loss of the ethanol and/or acetic acid. This disadvantage has not heretofore been recognized in the art.
This phenomenon of the ethanol bein present in the exhaust gas has been utilized in some presently available fermentators to monitor the fermentation process. The exhaust gases, those gases evaporating and/or passing through the wine, are directed through an analyzer. This analyzer measures the ethanol concentration in the air. As the concentration of the ethanol in the wine decreases the concentration of the ethanol in the exhaust gases correspondingly decreases. Once the ethanol concentration reaches a predefined level the fermentation process is stopped and a desired quantity of the fermentated wine is drawn out of the vessel. This concentration level is typically when the ethanol concentration in the wine, and in the air reaches about 0.3 volume percent. At this concentration level, the activity, and thus the concentration of the bacteria, in the fermentating wine is at a maximum. If the fermentation continues beyond this point, there exists the possibility of bacterial death due to the low level of ethanol in the substrate wine. This bacterial death creates the potential for foaming as discussed above. Thus it has been found that once this particular concentration level of the ethanol is achieved, the process should be terminated and the wine withdrawn from the vessel. After the exhaust gas has been analyzed, it is vented into the surrounding atmosphere.
In the Braun et al (3,252,870) patent reference, a cyclone separator is positioned in the exhaust line of the fermentator. This cyclone separator removes any liquid droplets present in the exhaust gas prior to the exhaust being introduced into a plate column gas washer. The exhaust gas is stripped of soluble vapors in this washer. However, the washing liquid containing the soluble components of the waste gas is discharged from the apparatus. Thus, the ethanol and/or acetic acid which is carried by the exhaust gas are not returned to the wine undergoing fermentation.
There thus remains a need to provide a fermentation process and useful apparatus which do not possess the disadvantages discussed above. A process and useful apparatus of this type should provide for a sufficient enough aeration and dispersal of an oxygen containing gas into the wine to initiate and maintain bacterial growth without the disadvantage of a substantial loss of ethanol and/or acetic acid. This type of process and useful apparatus should also provide for a multidirectional circulation of the wine to uniformly disperse the oxygen-bearing gas. Furthermore, the process of the invention should provide that the time necessary for successive fermentation should be reduced in order to provide a more economical system.